1. Field of the Invention
The present invention relates to an exhaust purification device of an engine, and more specifically to an exhaust purification device suitably applicable to a urea SCR (Selective Catalytic Reduction) system.
2. Description of Related Art
In recent years, a urea SCR system has been developed and has been partly put into practical use as an exhaust purification device for purifying NOx (nitrogen oxides) in exhaust gas at a high purifying rate in an engine (specifically, diesel engine) applied to a vehicle or the like. A following structure is known as a urea SCR system.
That is, in the urea SCR system, a SCR catalyst is provided in an exhaust pipe connected to an engine main body, and a urea solution addition valve for adding urea solution as a reducing agent into the exhaust pipe is provided upstream of the SCR catalyst. A urea solution tank is connected to the urea solution addition valve through a urea solution supply pipe. For example, if a pump provided in the urea solution tank is driven to discharge the urea solution, the urea solution is supplied to the urea solution addition valve through the urea solution supply pipe from the urea solution tank.
In this system, if the urea solution is added by the urea solution addition valve into the exhaust pipe, the urea solution is supplied to the SCR catalyst with the exhaust gas and the exhaust gas is purified by reduction reaction of the NOx on the SCR catalyst. When the NOx is reduced, the urea solution is hydrolyzed with exhaust heat to generate ammonia (NH3), which is added to the NOx in the exhaust gas selectively adsorbed by the SCR catalyst. The NOx is reduced and purified through the reduction reaction based on the ammonia on the SCR catalyst.
In the urea SCR system, the urea solution used as the reducing agent freezes, for example, at −11° C., and a trouble arises in the use of the urea solution because of the freeze. Therefore, as a countermeasure against the freeze of the urea solution, a technology of providing an electric heater to the urea solution supply pipe and of thawing the frozen urea solution in the pipe with the heater is proposed. For example, JP-A-2005-325691 describes covering an electric heater provided around an outer periphery of a pipe main body of a urea solution supply pipe with a plain-woven mesh, which is woven into a cylindrical shape with a fabric material, together with the pipe main body.
In the case of a conventional technology (including the above-described technology of JP-A-2005-325691) having a heater attached to a urea solution supply pipe, for example, the heater is energized for a predetermined period every time the engine is started, and electric power is consumed by the heater energization. If an in-vehicle battery is assumed as a power source in this case, a load applied to the battery can increase, causing an inconvenience. It is considered that such the inconvenience concerning the power consumption (increase in load of battery or the like) becomes remarkable in the case where the pipe length as the object of the urea solution thawing is long.